How to use the wire gauge and voltage drop calculator
- Choose AWG or mm² and select your wire size. Hook-up wire for robots is usually sold in AWG; Indian house wiring and many cables are sold in mm².
- Enter the one-way length — from the battery to the load. The calculator doubles it, because the current flows out on one wire and back on the other.
- Enter the current the load draws and the supply voltage.
- Leave the temperature at 20 °C unless the wire runs hot (inside an enclosure, near motors, or in summer heat).
- Read the voltage drop, the voltage left at the load, the power wasted in the wire and the smallest wire size that keeps the drop within your target (3 % by default).
Formula
- ρ – resistivity of copper, 1.72 × 10⁻⁸ Ω·m at 20 °C
- L – one-way length in metres, A – cross-section area in m² (1 mm² = 10⁻⁶ m²)
- n – AWG number (1/0 = 0, 2/0 = −1, 3/0 = −2, 4/0 = −3)
A useful shortcut: copper wire of 1 mm² has about 17.2 mΩ per metre. Halve the area and the resistance doubles.
Typical wire sizes
| AWG | Area | mΩ per metre | Typical use |
|---|---|---|---|
| 28 | 0.081 mm² | 212 | ribbon cable, signal wires |
| 24 | 0.205 mm² | 84 | breadboard jumpers, sensors |
| 22 | 0.326 mm² | 52.8 | hook-up wire, small gear motors |
| 20 | 0.518 mm² | 33.2 | robot motor leads |
| 18 | 0.823 mm² | 20.9 | battery leads, LED strips |
| 14 | 2.08 mm² | 8.27 | high-current battery and power wiring |
Resistance values are calculated for solid copper at 20 °C. The full table is inside the calculator.
Why voltage drop matters
Every wire is a small resistor. At low voltages the drop is a much bigger share of the supply: 0.5 V lost from a 230 V mains circuit is nothing, but 0.5 V lost from a 5 V supply is 10 %. That is why an ESP32 or Raspberry Pi Pico may reset when motors start, why LED strips look dimmer at the far end, and why motors lose speed and torque.
Common mistakes
- Using the one-way length only. The current goes out and comes back, so the resistance is for twice the distance.
- Assuming the wire is as thick as its label. Low-cost wire is often thinner than stated, or made of copper-clad aluminium, which has much higher resistance. If in doubt, choose one size bigger.
- Forgetting connectors. Breadboard contacts, screw terminals and thin JST leads add their own resistance on top of the wire.
- Ignoring temperature. Copper's resistance rises about 0.39 % per °C — about 16 % higher at 60 °C than at 20 °C.
- Mixing up AWG direction. A bigger AWG number means a thinner wire.
Related tools and guides
Wire loss also shortens battery life — estimate the runtime with the battery life calculator. For copper tracks on a circuit board, use the PCB trace width calculator, and for basic V = I × R maths try the Ohm's law calculator. Choosing a battery for your robot? Read the robot battery guide and the L298N robot car guide, or get everything in one of our robotics kits.
